Stuck in Traffic? Consult a Physicist

By MALCOLM W. BROWNE

Published: November 25, 1997

HATEFUL though they are to most people, traffic jams have fascinated some of the finest scientific minds of the 20th century, who see in them similarities to the freezing of water, the triggering of avalanches, the formation of galaxies and the advent of life itself.

Although physicists have made little progress in dissolving these clots in the arteries of automotive civilization, the enormous power of new computers has opened new possibilities that may one day reduce traffic congestion, improve the reliability of weather forecasting and solve other problems that so far have proved intractable.

The latest volley comes from Germany, where Dr. Boris S. Kerner, a research physicist at Daimler-Benz in Stuttgart, and Dr. Hubert Rehborn, a traffic consultant living in Aachen, have developed a theory that road traffic is subject to the kind of ''phase transitions'' that water undergoes when it abruptly changes to steam or ice.

Their paper, published last week in the journal Physical Review Letters, reports a series of measurements they have made since 1991 on a heavily traveled stretch of Autobahn highway near Frankfurt. They found that traffic along this German equivalent of a freeway flowed in three sharply differing modes: free flow, in which fast vehicles can change lanes and pass; ''synchronized'' flow, in which high traffic density prevents lane changes and passing, and jams, in which vehicles come to at least a momentary stop.

Although other physicists in the last 40 years have theorized about phase transitions in traffic, most have conceived of these transitions as a type known as ''second order,'' meaning that they occur gradually in response to gradual changes in average vehicle speed and traffic volume. But Dr. Kerner and Dr. Rehborn found, from data recorded by sensors under the roadway, that the phase transitions between free flow, synchronized flow and traffic jams were ''first order'' transitions. These occur abruptly and spontaneously, sometimes without any change in traffic volume or speed to trigger them.

Moreover, they found, when something interferes with freely flowing traffic, like an influx of slowly moving cars from a ramp, the resulting phase transition in the main stream of cars acquires a life of its own. A sluggish mass of synchronously moving vehicles propagates its infuriating mode of movement miles upstream from the obstruction and may persist for two or more hours, even after the obstruction is eliminated. In this, the clog seems to resemble a block of ice floating in water just above the freezing point: it neither grows nor rapidly melts.

Other investigators have shown that when traffic on a highway reaches about 85 percent of the highway's capacity, traffic becomes unstable; it may flow normally for a time, but it may congeal abruptly and without warning.

The German scientists concluded that the tendency of freely flowing traffic to ''self-organize'' spontaneously into synchronous flow or jams might reflect the influence of chaos, a condition in which tiny perturbations of a system's initial state can set off big changes in the system's evolution.

Chaotic sensitivity to initial conditions affects many phenomena, like weather or the frictional drag created by water flowing around a ship, and the effects of chaos are notoriously difficult to predict.

But if it was possible to create a vast network of sensors and traffic guidance systems under computer control, Dr. Kerner said in an interview, it should be possible to take a substantial bite out of congestion. A mechanism to delay a phase transition until after rush hour, Dr. Kerner said, would be a big improvement.

''Even though it's not possible to do this today, it's in our future,'' he said. Such a system would need to forecast accurately the occurrence of phase transitions that reduce traffic speed to a crawl.

''Traffic phase transitions are like cancer,'' he said. ''Once a transition occurs, it may be too late to fix. The answer is timely and accurate forecasting of an approaching phase transition.''

Dr. Kerner offered no specific suggestions for blocking incipient phase transitions. In general, he said, a neural network of sensors, a powerful computer traffic analysis system and responsive highway controls of some kind would be needed.

Before much can be done about jams in a practical way, physicists believe that they need to understand the fine details of the physics underlying traffic dynamics. To that end, the Federal Government is conducting studies in the Dallas-Fort Worth area and in Portland, Ore.

With financing from the Department of Transportation, physicists at Los Alamos National Laboratory in New Mexico plan to spend $25 million over seven years to study traffic in the two urban areas. Their aim is to use this information to create computer simulations that mimic the behavior of real traffic.

''The beauty of this thing,'' said Dr. Richard J. Beckman, a project leader at Los Alamos, ''is that you can try out a policy decision before you actually implement it. For example, you can ask the simulation to show what would happen if you add three new lanes to a freeway.''